1. Field of the Invention
The present invention relates to a polymer producing method, a polymer producing apparatus, a polymer degassing method, and a polymer degassing apparatus. The present invention particularly relates to a technique for removing unreacted monomers and the like from a ring-opening polymerized polymer such as polylactide.
2. Background Art
Polylactide, which is a polymer synthesized by a ring-opening polymerization reaction, is a colorless and transparent polyester prepared using, as a raw material, lactic acid, which is a hydroxycarboxylic acid. A known method for synthesizing polylactide from lactic acid is a method wherein lactic acid is condensed to produce an oligomer, a catalyst such as antimony oxide is added to the oligomer to carry out thermal cracking to produce lactide (cyclic dimer of lactic acid), and a catalyst such as tin octylate is added to the lactide to carry out ring-opening polymerization.
For instance, a widely known method for polymerizing polylactide is a continuous method in which reaction tanks for ring-opening polymerization of lactide are connected in series, and a raw material, i.e., molten lactide, is supplied and a polymerized product is discharged at the same time. In such a case, operations are carried out by adequately controlling the temperature, the amount of catalyst, and the residence time, which may be varied for each reaction tank. Thus, it is possible to inhibit yellowing as a result of thermal decomposition of polylactide due to prolonged temperature history and accumulation of reaction heat associated with polymerization.
As an aside, polylactide which has been produced in the above manner contains unreacted lactide in large amounts. Thus, it is fragile in terms of strength and tends to be hydrolyzed, which is problematic. Therefore, an apparatus for use in removing unreacted lactide in a vacuum pressure environment from polylactide obtained as a final polymerized product (demonomerization) has been disclosed (see JP Patent No. 3419609). In the case of the apparatus disclosed in JP Patent No. 3419609, a stirrer having a rotation shaft to which a plurality of stirring blades have been attached in parallel at a certain interval is accommodated in a horizontal container, and the container has a port for supplying a polymerized reactant (polymer), a port for discharging the same, and a degassing port for degassing a gas. The reactant supplied to the container is transferred within the container as the stirring blades rotate and the reactant is removed therefrom in a state in which an unreacted monomer is degassed in a vacuum atmosphere.
In addition, JP Patent Publication (Kokai) No. 10-218998 A (1998) discloses an invention concerning a technique for obtaining polycondensed polyester in which a stirring body is formed by alternately orienting a plurality of blade plates and doughnut-shaped circular plates longitudinally with respect to a container. The stirring body is allowed to rotate circumferentially within the container in order to promote a polymerization reaction, and a by-product is removed by degassing. According to this technique, stirring is carried out on a face oriented substantially orthogonally with respect to the flow of a reaction solution in the longitudinal direction of the container (vertical face) and no stirring shaft exists at the virtual rotation center, such that polymer deposition on a rotation shaft can be inhibited.
Likewise, JP Patent Publication (Kokai) No. 2004-10791 A discloses an invention concerning a stirrer in which two rotation blade bodies, each having no stirring shaft inserted through the virtual rotation center, are arranged in parallel in a container and they are allowed to rotate in opposite directions. According to this apparatus, stirring is carried out with a spectacle-shaped stirring blade on a face oriented substantially orthogonally with respect to the flow of a reaction solution flowing longitudinally in the container and no stirring shaft inserted through the virtual rotation center exists, such that polymer deposition on a rotation shaft can be inhibited.
Further, JP Patent No. 2523770 discloses an invention concerning a technique for obtaining polycondensed polyester in which stirring blades formed by coupling rectangular stick frames oriented longitudinally with respect to a tank are arranged in parallel in a container and they are allowed to rotate in the container. According to this technique, stirring is carried out along with rotation of rectangular frames that do not have faces oriented substantially orthogonally with respect to the flow of a reaction solution flowing longitudinally in the container, such that polymer deposition on a rotation shaft inserted through the virtual rotation center or on the stirring blade surface can be inhibited.
Meanwhile, JP Patent Publication (Kokai) No. 2005-220203 A discloses a technique comprising a continuous ring-opening polymerization step for obtaining mainly polylactide, in which at least two reaction tanks are connected in series, provided that a horizontal polymerization tank is placed in the fore stage and a vertical polymerization tank is placed in the rear stage, and a residual monomer is removed following the ring-opening polymerization reaction. According to this technique, a reaction solution has plug-flow characteristics and a high capacity to remove reaction heat in the ring-opening polymerization step. Thus, polymerization can proceed to achieve high reactivity with less thermal degradation. Accordingly, a high-quality polymer can be synthesized.
Furthermore, JP Patent Publication (Kokai) No. 2007-126601 A discloses a technique for mixing an additive with a reaction solution between a continuous ring-opening polymerization step for obtaining mainly polylactide and the subsequent step of removing a residual monomer. According to this technique, a depolymerization reaction is inhibited in the step of removing the residual monomer and thus a substantially degradation-free and high-quality polymer can be synthesized.